Ruggedized electronics enclosure

ABSTRACT

The present invention relates to a ruggedized enclosure for housing and protecting electronic circuits. The enclosure utilizes a top compartment for housing the circuit and a cooling assembly rigidly coupled to the top compartment. The cooling assembly utilizes a passive radiator to form a rigid truss plate structure. The truss plate structure rigidifies the enclosure helping to protect the enclosure and circuit from destructive shock events and destructive vibration events. The cooling assembly further provides an efficient heat exchange for removing heat from the electronic circuit. A method for protecting an electronic circuit utilizing a rigid truss plate structure is also provided.

RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.10/850,523, entitled “Ruggedized Electronics Enclosure”, that was filedon May 19, 2004, which is a continuation of U.S. patent application Ser.No. 10/232,915, entitled “Ruggedized Electronics Enclosure,” that wasfiled on Aug. 30, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention is related to enclosures for electronic circuits andparticularly to ruggedized enclosures for use in installations subjectedto hostile environments including destructive shock events anddestructive vibration events. In one embodiment, the invention mayoperate without requiring additional mechanical isolation.

2. Description of the Related Art

Conventional ruggedized electronics enclosures are often employed inmilitary applications. The environments in which military electroniccircuits must be able to operate typically present conditions outside ofa commercial electronic circuit's operational parameters. Examples ofsuch conditions include excessive moisture, salt, heat, vibrations, andmechanical shock. Historically, military electronic equipment was custommade to provide the required survivability in the hostile environments.While effective in surviving the environment, custom equipment is oftensignificantly more expensive than commercial systems, and is typicallydifficult if not impossible to upgrade to the latest technologies.Therefore, a current trend in conventional military hardware is to adaptcommercially available electronics for use in military applications.These systems are typically known as Commercial Off The Shelf systems,or COTS.

The COTS design philosophy has allowed the military to keep current withtechnological innovations in computers and electronics, withoutrequiring specialized and dedicated electronic circuit board assemblies.The COTS design methodology is attractive because of the rapidlyincreasing computational power of commercially available,general-purpose computers. Since the components in a COTS system arecommercially available, though usually modified to some extent, themilitary can maintain an upgrade path similar to that of a commercial PCuser. Thus the COTS philosophy allows the military to integrate the mostpotent electronic components available into their current hardwaresystems.

While COTS systems have allowed the military to reduce the cost ofequipment and to make more frequent upgrades to existing equipment,there are inherent disadvantages to COTS systems. As noted above,military applications must be able to withstand various environmentalextremes, including humidity, temperature, shock and vibration. Theseconditions are typically outside of the operating parameters ofcommercial electronics and, thus, added precautions and modifications tothe physical structures of the equipment must be made to ensurereliability of operation in these environments. Conventional COTSsystems typically use two specialized modifications to maintainreliability. These approaches may be used separately, or in combination.

To deploy COTS equipment in hazardous environments, COTS components arehoused in a complex ruggedized enclosure or case. One approach,sometimes referred to as “cocooning” places a smaller, isolatedequipment rack within a larger, hard mounted enclosure. With thisapproach shock, vibration and other environmental extremes areattenuated by the isolation system to a level that is compatible withCOTS equipment. Another approach, sometimes called Rugged, Off The Shelf(ROTS) seeks to “harden” the COTS equipment, in a manner such as to makeit immune to the rigors of the extended environmental conditions towhich it is exposed. This later approach strengthens the equipment'senclosure and provides added support for internal components. Bothcocooning and ROTS design methodologies must also improve coolingefficiency to accommodate higher operating ambient temperatures. Bothapproaches suffer from added complexity, size, weight and cost.

The size and complexity exacerbates heat-removal from the enclosure andoften complex heat flow routes must be devised in order to maintain adesirable operating temperature. Taken together, these designconsiderations drastically increase the cost and complexity of such anenclosure.

Commercial systems are typically designed around three main criteria,cost, time-to-market and easy expansion. To deliver on all three designgoals, the assumption is that the environment for the system will not beexposed to extreme environmental conditions. Cost is the primarymotivator to keeping the packaging simple and inexpensive. The packagesupport structures may have a low cost to keep the system cost fromescalating. Keeping costs down to a minimum is counter to therequirements of making a system robust enough to survive a militaryenvironment.

To easily accommodate system expansion, computer manufacturers try tosimplify the installation of peripheral cards, memory and storage. Theidea of having a minimum number of fasteners (i.e., a snap-in-placedesign) allows the customer easy access and installation of peripherals.The design's modularity preserves the customer's investment. When youcouple the commercial constraints with the requirements of the militaryenvironment, the design requires a different approach, typically movingthe structural changes to the system enclosure and it's attachments. Theusual cocooning approach is to design the enclosure to absorb as much ofthe shock as possible to allow the incumbent system to survive theenvironment. In practice, this is not easily achieved, especially whenusing larger, and heavier computer systems. Thus, the idea of completelyisolating a commercial system from the rigors of the militaryenvironment is difficult to achieve and adds a large cost premiumbecause the rack is the item being modified. The current solution tosupporting COTS technology in a military environment described above,adds significant complexity to the system.

Two of the most difficult conditions to design for are vibration andmechanical shock. Mechanical shock and vibration may over time destroyelectronic equipment by deforming or fracturing enclosures and internalsupport structures and by causing electrical connectors, circuit cardassemblies and other components to fail. In military applications, aswell as in commercial avionics and the automotive industry, electronicsmust be able to operate while being subjected to constant vibrationalforces generated by the vehicle engines, or waves, as well as beingsubjected to sudden, and often drastic, shocks. Examples of such shocksare those generated by bombs, missiles, depth charges, air pockets,potholes, and other impacts typically encountered by military orcommercial vessels. Furthermore, these conditions may also be seen inthe operating conditions of a network or telephone server during anearthquake. While providing some protection from shock and vibration,the conventional ruggedized enclosure operating alone cannot provideadequate protection for mission-critical electrical components andcircuits.

In order to provide additional protection against shock and vibration,conventional COTS systems mount the ruggedized enclosures describedabove in a mechanically isolated cocoon. FIG. 1 illustrates aconventional mechanically isolated cocoon. As illustrated in FIG. 1, acocoon 100 is provided to house the various ruggedized enclosures 110.The cocoon 100 may be attached to a floor 130 and/or a wall 140 of itssurroundings. Commonly this includes the fuselage or deck plate of amilitary vehicle. The cocoon 100 is attached to the surroundings 130,140 via mechanical isolators 120. A particularly advanced mechanicalisolator 120 is the polymer isolator illustrated in FIG. 1, thoughconventional systems may use any spring-like apparatus to provide theisolation. By attaching the cocoon 100 to its surroundings 130, 140 viamechanical isolators 120, the cocoon 100 is allowed limited movementwith five degrees of freedom. This limited movement helps to dampen theeffects of shock and vibration.

There are several drawbacks to using the mechanically isolated cocoon100. In order to reduce the shock to the equipment, the cocoon 100 mustbe provided with a sway space 150 in which it may move unobstructed.Typically this sway space 150 is four to seven inches in each directionof movement. Thus the cocoon 100 consumes additional space 150 whichmight otherwise be utilized for other activities or equipment. Inmilitary applications, commercial aircraft, as well as automotiveapplications, space is often at a premium.

Additionally, while the cocoon 100 does isolate the equipment from somevibration and shock, it does not completely isolate the equipment. Forexample, a conventional cocoon 100 can receive a 60-80 G shock (a “G” isa unit of force equal to the force exerted by Earth's gravity on a bodyat rest and is used to indicate the force the equipment is subjected towhen accelerated by a shock event) and reduce the shock felt by theequipment to 10-15 G's. Typically the performance of the cocoon 100 islimited by sway space available, materials used, and equipment placementwithin the cocoon 100. Additionally, if the environment around thecocoon 100 moves more than the sway space 150 can accommodate, then thecocoon 100 and its equipment will feel the entire shock event. While asignificant reduction in the shock may be experienced, it is importantto note that commercial equipment is frequently rated for 5 G's or less.Thus, there is still a significant chance for failure within the system.

To provide the additional shock protection, conventional COTS systemspair the cocoon 100 with the ruggedized enclosures 110, or cocoon.However, while more effective in protecting the equipment frommechanical shock, these ruggedized enclosures 110 work only when theshock isolation system is carefully integrated with the includedsystems. Since the enclosure is allowed to move, issues such as weight,position, center of gravity and heat removal all have to be balanced.Thus, the cost and complexity of such a system are significantly higherwhen compared to a commercial system using similar electricalcomponents.

What is needed is a ruggedized enclosure for use in hostile environmentswhich: 1) provides a simplified and effective heat flow design; 2) mayutilize COTS components; 3) does not require the use of a mechanicalisolator or sway space; 4) provides a high level of shock and vibrationprotection without need for augmentation; and 5) may be manufactured atlow cost.

SUMMARY OF THE INVENTION

The present invention overcomes the limitations and disadvantages ofconventional electronics enclosures used in harsh operatingenvironments. In one embodiment, the invention provides protection fromdestructive shock events and destructive vibration events without needof external mechanical isolation.

In one embodiment, the electronics enclosure includes a top compartmentfor housing the electronic circuit, and a cooling assembly attachedthereto. The top compartment may be sealed to further protect theelectronic circuit from moisture and unwanted particles in the air. Thecooling assembly includes a rigid truss plate structure which forms astructural member for rigidifying the enclosure, and also forms anefficient heat radiator for removing heat from the electronic circuit.The truss plate structure achieves it's high strength to weight ratio ina manner similar to conventional “honey-comb” or sandwich structures.The truss plate structure converts bending mode forces, applied toopposing plates, into compression and extension mode forces. However,unlike conventional “honey-comb” or sandwich constructions, the presentinvention provides ducts or passage ways through which cooling air (orother cooling fluid) is allowed to flow to aid in the efficient removalof heat from the top compartment. In an alternate embodiment, the trussplate structure is a honey-comb truss structure that provides passagesthrough which cooling air (or other cooling fluid) is allowed to flow.

In one embodiment, the rigid truss plate structure is formed from apassive radiator coupled between a heat spreader plate and a bottomplate. The heat spreader plate also forms the bottom of the topenclosure and provides both mechanical and thermal coupling between thetop compartment and the cooling assembly. In one embodiment, the passiveradiator may be comprised of a corrugated fin. In another embodiment,the passive radiator is comprised of triangularly shaped fins (anA-frame structure). Both the corrugated fin and the triangular finstructure may provide additional protection against destructive shearand twisting of the enclosure. In another embodiment, the passiveradiator is comprised of a pin-style heatsink. In one embodiment thepin-style heatsink is arranged according to a pin density pattern tocreate a turbulence gradient for the cooling assembly.

In one embodiment, the enclosure is rigidified by the truss platestructure in order to protect the electronic circuit against ananticipated destructive shock event. In one embodiment, the enclosureand circuit can withstand and survive a 60 G shock event. In alternateembodiments the enclosure is designed based upon various criteria suchthat a particular enclosure and enclosed device (e.g., circuit) isdesigned to withstand and survive shock events in the range of 20 G toat least 60 G depending upon these design criteria. In anotherembodiment, the enclosure's resonant frequency is raised above ananticipated destructive vibration event. In one embodiment, of specialinterest for land vehicle or aircraft applications, the enclosure andcircuit have a resonant frequency in the range of 200 Hz to at least 1kHz. In another embodiment, of special interest for shipboardapplications, the enclosure and circuit have a resonant frequency in therange of 20 to 40 Hz. The listed ranges are merely exemplary, andalternate embodiments may have a resonant frequency selected to behigher than a known destructive vibration event.

In one embodiment, the cooling assembly further provides heat pipes fordrawing away additional heat from the electronic circuit and deliveringit to an external heat exchanger. In one embodiment, the heat pipescooperate with the passive radiator to provide an efficient heatexchanger.

In one embodiment, the electronic enclosure includes the use ofmicrochips. These chips may be placed top-down on the heat spreaderplate in order to provide a more efficient heat transfer from the chipto the cooling assembly.

A method for protecting and cooling an electronic circuit via a rigidtruss plate structure is also provided.

The features and advantages described in the specification are not allinclusive, and particularly, many additional features and advantageswill be apparent to one of ordinary skill in the art in view of thedrawings, specification and claims herein. Moreover, it should be notedthat the language used in the specification has been principallyselected for readability and instructional purposes, and may not havebeen selected to delineate or circumscribe the inventive subject matter,resort to the claims being necessary to determine such inventive subjectmatter.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a conventional mechanically isolated cocoon system.

FIG. 2 illustrates an exploded view of a ruggedized electronicsenclosure according to one embodiment of the present invention.

FIG. 3 illustrates a cut-away structural detail of the assembledruggedized electronics enclosure according to one embodiment of thepresent invention.

FIG. 4 illustrates a cut-away diagram of the ruggedized electronicsenclosure showing heat and airflow related to the enclosure according toone embodiment of the present invention.

FIG. 5 illustrates a cooling assembly utilizing a triangular finstructure.

FIG. 6 illustrates a cooling assembly utilizing a pin-style heatsink.

FIG. 7 illustrates a cooling assembly utilizing a pin-style heatsinkforming a turbulence gradient.

DETAILED DESCRIPTION

A preferred embodiment of the present invention is now described withreference to the figures where like reference numbers indicate identicalor functionally similar elements. Also in the figures, the left mostdigit(s) of each reference number correspond(s) to the figure in whichthe reference number is first used.

The present invention relates to a ruggedized electronics enclosure forprotecting electronic circuits that must be able to survive and operateunder harsh conditions such as those in military and automotiveenvironments. The enclosure must be able to protect the electroniccircuits from severe vibration and shock, heat, moisture, dustparticulate, and various other adverse conditions. Throughout thisdescription, the word “destructive” will be used to indicate a force orevent which may cause the enclosure or the electronic circuit to failafter a single occurrence of the event, or after repeated occurrences ofthe event between maintenance intervals. Specific destructive eventswill be discussed in more detail below.

FIG. 2 illustrates an exploded view of a ruggedized electronicsenclosure 200 according to the present invention. As illustrated in FIG.2, the enclosure 200 is configured to house and protect a computeelement 210. The compute element 210 is chosen by way of example asillustrative of the primary features and operation of the enclosure 200,and one skilled in the art will recognize that the enclosure 200 may beconfigured to house and protect any electronic circuit. Examples ofalternate electronic circuits include various other components used in acomputer, ordinance guidance and communication boards, vehicle controlmodules, radio and communications equipment, radar equipment, etc. Aswill be discussed below, the enclosure 200 may most advantageously beused for any electronic circuit which may be formed having a lowvertical profile, but may be used to add increased protection to anydimensioned electronic circuit.

The ruggedized electronics enclosure 200 includes a top compartment 220for housing the electronic circuit 210 (illustrated as a computeelement), and a cooling assembly 230 coupled to the bottom of the topcompartment 220. As illustrated, the enclosure 200 is shaped as arectangle, however any footprint shape may be used. Non-rectangularshapes may be preferred in applications where space is at a premium,such as in aircraft, or military ordinance.

The top compartment 220 includes a top cover 222, one or more thermalinterposers 224, a pair of side walls 226, a front wall 227, a rear wall(not shown) and a heat spreader plate 240. In one embodiment, the sidewalls 226, front wall 227 and rear wall as well as the top cover 222 areformed from aluminum. Alternatively, these portions of the topcompartment 220 may be may be formed of any rigid material including,but not limited to steel, and plastics. Preferably, side walls 226 aresized to extend the entire combined height of the top compartment 220and cooling assembly 230. Front wall 227 and rear wall are preferablysized to extend the height of the top compartment 220. An upper portionof side walls 226, front wall 227, the back wall, top cover 22 and heatspreader plate 240 cooperate to form the sealed top compartment 220 forhousing the electronic circuit 210. In another embodiment, the topcompartment 220 may not be sealed, but may instead be open to theenvironment. The various parts which form the top compartment 220 may becoupled together using screws or other fastener types that may requirespecial tools for removal. Additionally, the screw fasteners may beaugmented by other self-aligning/locking mechanical components. Byutilizing screw fasteners or other removable fasteners, the topcompartment 220 may be opened as necessary to provide service to theelectronics housed inside. Alternatively, the compartment structures, ora substructure therein, may be formed by milling or casting a singlepiece of material such as aluminum, steel or plastic. Anotheralternative includes welding the elements comprising the top compartment220 together to form a solid enclosure. However, while welding mayincrease structural stability, it decreases the enclosure's 200serviceability.

The cooling assembly 230 is coupled to the bottom of top compartment 220and further includes a passive radiator 232 (here illustrated in oneembodiment 232 a) and a bottom plate 234. The passive radiator 232 andbottom plate 234 are coupled to the cooling assembly 230 in order todraw heat away from the highest dissipation components (the topcompartment 220) to a high efficiency heat exchanger (the passiveradiator 232).

As illustrated, the passive radiator 232 may be formed from an aluminumcorrugated fin 232 a. As will be discussed below, the use of an aluminumcorrugated fin 232 a provides specific advantages over other passiveradiators, however, one skilled in the art will recognize that otherpassive radiators may be used in place of the corrugated fin 232 a, aswell as that the radiator 232 may be made from other material aside fromaluminum. For example, the passive radiator may be formed from copper,carbon fiber, composite structures of aluminum and copper or plastic,and may additionally be used in conjunction with heat-pipes and coldplates. Additionally, other structures aside from a corrugated fin 232 amay be used. FIG. 5 illustrates a triangular fin, or A-frame, trussstructure 232 b preferably formed from aluminum or steel. As will bediscussed below, this embodiment of the passive radiator 232 is moredifficult and more expensive to manufacture, but provides additionalstructural integrity to the enclosure 200. FIG. 6 illustrates anotherembodiment of the passive radiator 232 utilizing pin-style heat-sinks232 c sandwiched between the heat spreader plate 240 and the bottomplate 234. This forms a rigid truss plate structure while allowing somemeasure of heat dissipation profiling based on the placement and densityof the pins.

In general, heat spreader plate 240, a lower portion of side walls 226,and bottom plate 234 cooperate to “sandwich” the passive radiator 232into a solid rigid truss plate structure. The truss plate structureachieves a high strength to weight ratio by converting bending modeforces, applied to opposing plates, into compression and extension modeforces. This is similar to plates formed from conventional honey-comb orsandwich construction. However, unlike conventional “honey-comb” orsandwich construction, the present invention provides ducts orpassageways through which cooling air (or other cooling fluid) isallowed to flow to aid in the efficient removal of heat from the topcompartment 220.

The cooling assembly 230 may be assembled in a number of ways, with onegoal being to keep the assembly process simple, while preservingstructural rigidity and allowing the effective transfer of heat from thebase-plate to the passive radiator 232. One way of doing this with ametallic passive radiator 232 is through welding. If a non-metallicpassive radiator 232 is used, a thermally conductive adhesive may beused.

As illustrated, electronic circuit 210 is a compute element and includesa PCB 212, a plurality of processors 214 coupled to a front of the PCB212, and a plurality of memory components 216 electrically coupled to aback of PCB 212. A thermal interposer 224 a is positioned to contact theback of PCB 212 and the memory components 216 to provide a heat exchangebetween PCB 212 and memory components 216. Typically, the interposer 224is made up of a resilient plastic material, doped with a thermallyconductive and insulating compound such as aluminum oxide, boron nitrideor other materials. Alternatively, the interposer 224 may be formed froma gel or a foam. Alternatively, the top compartment 220 may be filledwith thermally conductive foam. While this alternative providesstructure and heat removal, it is not preferred due to the permanentnature of the installation. A removable interposer 224 is preferred toaid in the keeping the electronics inside the top compartment 220serviceable.

As will be discussed in greater detail below, processors 214 and PCB 212are positioned within top compartment 220 such that processors 214 areplaced in physical contact with heat spreader plate 240, allowing forheat to be conducted away from processors 214. Alternatively, a heatconducting material, such as a thermal interposer similar to interposer224, may be position between the processors 214 and the heat spreaderplate 240. A second thermal interposer 224 is positioned between thememory components 216 and the top cover 222. Top compartment 220 ispreferably sized to provide just enough vertical and horizontal room tofit electronic circuit 210 within its confines. In a preferredembodiment, thermal interposers 224 are created from a resilientmaterial which is slightly compressed to ensure a “snug” fit for theelectronic circuit 210 within top compartment 220. By ensuring that thethermal interposers 224 make tight contact with the top cover 222,additional thermal and structural benefits are realized.

FIG. 3 illustrates a cut-away structural detail of the assembledruggedized electronics enclosure 200. As introduced in FIG. 2, in oneembodiment, the electronic circuit 210 housed in the top compartment 220is again a compute element. One of the objectives for the ruggedizedelectronics enclosure 200 is to provide protection to the electroniccircuit 210 housed in the top compartment 220 from harsh operatingenvironments. As noted above, the top compartment 220 may be completelysealed by appropriately sizing the side walls 226, front wall 228 (notshown), back wall (not shown), top cover 222 and heat spreader plate 240to ensure that no open spaces exist in the top compartment 220 surface.

In addition to being able to make the top compartment 220 airtight,additional steps may be made to “ruggedize” the enclosure 200 to helpreduce the effects of destructive shock events and destructive vibrationevents on the electronic circuit 210 housed within. A destructive shockevent is any shock event that may render the electronic circuit 210 orenclosure 200 inoperative due to a large change in force and momentumbeing applied to the circuit 210 and enclosure 200. The circuit 210 orenclosure 200 may be rendered inoperative after a single destructiveshock event or after a series of destructive shock events occurringbetween maintenance intervals. Examples of destructive shock eventsinclude impacts and explosions from bombs, missiles, other militaryordinance, water craft hitting depth charges, aircraft hitting airpockets, wheeled vehicles hitting potholes as well as other impactstypically encountered by military or commercial vessels. One skilled inthe art will recognize that other destructive shock events exist andthat the above list provides only a general context for the nature of adestructive shock event.

Similarly, a destructive vibration event is any vibration event that maycause the electronic circuit 210 or enclosure 200 to fail due to aweakened structural integrity. Destructive vibration events may beisolated and short-lived in duration or may always be present in theoperating environment. Examples of destructive vibration events includeengine vibrations, turbine vibrations, screw vibrations, prolonged shockevents, travel along uneven surfaces etc. One skilled in the art willrecognize that other destructive vibration events exist and that theabove list provides only a basic context for the nature of a destructivevibration event.

In typical military applications, the electronic circuit 210 must beable to survive and continue to operate efficiently after beingsubjected to an 60 G shock or constant vibration from engines and othermovement. Military specifications MIL810, MIL901, MIL167 and ISO10055provide specific requirements for shock and vibration resistancedepending on the desired application and are incorporated in theirentireties herein. Typically, the individual chip-level components usedin a standard commercial environment will withstand up to an 60 G shockload. This is due in part to the fact that the interconnects and siliconare packaged such that there is high structural rigidity in thecomponent. However, one concern is with the printed circuit board (PCB)and its assembly. To minimize the shock impact to the PCB and the solderconnections, it is beneficial to have structural ties between the boardand its components and cooling assembly 230.

One design goal is to make the entire enclosure assembly one rigidstructural element in order to protect against destructive shock andvibration events. In one embodiment, the enclosure is rigidified by thetruss plate structure in order to protect the electronic circuit againstan anticipated destructive shock event. In one embodiment, the enclosureand circuit can withstand and survive a 60 G shock event. In alternateembodiments the enclosure is designed based upon various criteria (e.g.,materials, mass, truss plate, dimensions, assembly methods, etc.) suchthat a particular enclosure and enclosed device (e.g., circuit) isdesigned to withstand and survive shock events in the range of 20 G toat least 60 G depending upon these design criteria.

One aspect of forming the enclosure 200 as a rigid structural elementincludes raising the enclosure's 200 resonant frequency to a frequencyhigher than the destructive vibration events to which the enclosure 200,will be subject. Two major factors that affect the resonant frequency ofa given structure are the mass, and the material's inherent stiffness.Typically, the lower the mass, the higher the resonant frequency. Thus,the overall mass of the enclosure 200 helps determine the resonantfrequency of the enclosure 200 as well as its susceptibility tovibrational damage. Also, the higher the material stiffness, the higherthe resonant frequency. As noted above, from a vibration standpoint, itis desirable to have the resonant frequency above the frequencies of anyanticipated destructive vibration events to keep the mechanicalstructure from adding to the vibration energy.

Thus, the enclosure 200 is formed from a material that balancesstiffness and mass to provide an overall high resonant frequency whichis higher than the anticipated destructive vibration event frequencies.In the preferred embodiment, the ruggedized enclosure 200 is composedprimarily of aluminum. The use of aluminum offers a good compromisebetween strength needed to protect the electronic circuit 210, whileproviding a lower total mass for the enclosure. As will be discussedbelow, the use of aluminum also provides an efficient way of removingheat generated by the electronic circuit 210. In one embodiment, theenclosure 200 is designed to have a resonant frequency that is at leastapproximately twice the 12-25 Hz frequency of naval shock events. In analternate embodiment, the enclosure 200 has a resonant frequency in therange of hundreds of Hz, to protect the enclosure against an aircraft'sprop or turbine vibrations. The specific resonant frequency chosen willbe dictated by the specific vibrational frequency of the prop or turbineengine used, e.g., between 200 Hz and 1 kHz. These frequencies aremerely examples of the resonant frequencies supported by the presentinvention. Alternate embodiments will have a resonant frequency selectedto be greater than the vibrational frequency of an anticipated shockevent that is to be dissipated by the enclosure 200.

Another aspect of the ruggedized enclosure 200 is its overall profile.In a preferred embodiment, the overall vertical height of the enclosure200 is 1 rack unit (“U”) or 1.75 inches. Additionally, in oneembodiment, the top compartment 220 is configured to house theelectronic circuit 210 snugly, without allowing for significanthorizontal or vertical movement within the compartment 220. Furthercushioning and insulation from vibration is garnered by the use of thethermal interposers 224 which may be compressed slightly to ensure asnug fit while providing an efficient heat conduit to remove heat fromthe electronic circuit 210.

Passive radiator 232 provides additional resistance to destructive shockand vibration events. By using a passive radiator and fluid channelstructure such as the corrugated fin 232 a, the triangular fin 232 b, orthe pin-style heatsink 232 c, a light-weight rigid truss plate structuremay be formed from the cooling assembly 230. This structure is stiffenedby cross coupling (via the passive radiator 232) between the topcompartment 220 and bottom plate 234. By forming the truss platestructure, the passive radiator 232 provides the cooling assembly 230with structural properties similar to a solid thick plate from arigidity standpoint for resisting destructive shock and vibrationevents. While a solid thick plate generally provides additionalstructural integrity to the enclosure 200, there is a tradeoff betweenplate thickness and overall mass. As noted above, the resonant frequencyof the enclosure 200 would be decreased by the increased mass of a solidplate. By instead using a truss plate structure for the cooling assembly230, the enclosure 200 retains the benefit of a thick plate whileavoiding the lower resonant frequency associated with a thick, heavyplate.

In addition to the passive radiator 232, the interposers act to absorbhigh frequency vibrations by acting as lossy dissipative elements. Thecombination of top cover 222, thermal interposers 224, electroniccircuit 210, and cooling assembly 230 in a small vertical space helpsmakes the total enclosure 200 very stiff. Furthermore, the interposersreduce the transfer of energy between the bottom plate 234 and the topcover 222, essentially dissipating the conducted- vibrational energy.Additionally, materials used in bottom plate 234, heat spreader plate240 and top cover 222 may be selected to dissipate mechanical(vibrational) energy. In particular, composite materials can offer acombination of high strength (stiffness) and damping (mechanical energydissipation).

As noted above, the truss plate structure helps rigidify the enclosure200 by cross coupling the top compartment 220 and the bottom plate 234.For example, the use of the triangular fin structure 232 b or corrugatedfin 232 a as the passive radiator 232 may also help reduce the effectsof destructive shear events and destructive vibration events in thehorizontal direction indicated by arrow 310 and in a vertical directionindicated by arrow 320. Using a corrugated fin 232 a for the passiveradiator 232 provides a good structure to transfer energy in bothhorizontal and vertical direction. The corrugation directs forces alongthe axes of the structure. The corrugations may also act to reduce thevibrational energy by acting as a dissipative spring. Tying thecorrugations to the top and bottom plate 240, 234 at the peaks stiffensthe structure in the “vertical” direction, effectively raising thestructure's vertical (or bending mode) resonant frequency.

FIG. 4 illustrates a cut-away diagram of the ruggedized electronicsenclosure showing heat and airflow related to the ruggedized electronicsenclosure 200. In FIG. 4, to more clearly illustrate the heatflow andairflow, the top compartment 220 is not fully shown, but it isunderstood that the cooling assembly 230 is coupled to a top compartment220 which houses and protects electronic circuit 210 as illustrated inFIG. 2.

FIG. 4 illustrates two directions for heat flow from electronic circuit210, here illustrated as PCB 212 and processor 214. A primary directionfor heat flow is illustrated by an arrow 410. This heat flow isaccomplished by putting the processor 214 in thermally conductivecontact with heat spreader plate 240. In one embodiment contact may bemade by placing the processor 214 in direct contact with the heatspreader plate 240. Alternatively contact may be made by placing a heatconductive medium between the processor 214 and the heat spreader plate240. Preferably, heat spreader plate 240 has a high thermalconductivity. In a preferred embodiment, processor 214 is oriented to beupside down so that its “top” is pressed against heat spreader plate240. This arrangement allows for direct heat conduction betweenprocessor 214 and heat spreader plate 240. In conventional microchips,the main direction for heat to escape the chip is through its “top”. Bypositioning the top of the processor 214 against the heat spreader plate240, heat is efficiently conducted from the processor 214 to the heatspreader plate 240. Alternatively, the microchips may face with their“tops” away from the heat spreader plate 240 and a thermal interposer224 or other thermally conductive medium may be placed between themicrochip and the heat spreader plate 240.

Heat spreader plate 240 conducts heat away from the electronic circuit210 in the direction indicated by arrow 410, and into the passiveradiator 232. Passive radiator 232 is designed to radiate the heatconducted from the electronic circuit 210 into the environment.Preferably, passive radiator 232 is exposed to an air flow across itssurface area. This air flow is indicated by arrow 430 in FIG. 4. Byinducing an air flow 430 through the spaces formed from passive radiator232 and top and bottom plates 240, 234, heat may be efficiently removedfrom the electronic circuit 210 and from the ruggedized electronicenclosure 200 in general. Alternatively, the cooling assembly 230 can bemounted vertically to allow the heated air to rise, cooling the assemblythrough thermally induced convection currents. The specific proportionsof passive radiator 232 directly affect its efficiency in removing heatfrom the enclosure 200. For instance, the overall height and width of asingle “segment” directly affects the amount of surface area present forradiating heat, as well as changing the profile of the air channels. Theprofile of the air channels affects the channel's impedance to airflowand thus, the rate of airflow (for a given pressure differential)through the air channels of the passive radiator 232 and consequentlythe enclosure 200.

Additionally, for low airflow situations, the cooling assembly 230 isdesigned to radiate the maximum amount of heat to the ambient air.Increasing the surface area increases the heat transfer between theprocessor and the air. This may result in a “tighter” corrugation ormore transitions between the heat spreader plate 240 and the bottomplate 234. If, however, an externally generated pressure differential isused to induce air movement past the passive radiator 232, then thedesign may optimize the passageways through the passive radiator 232 foroptimum heat transfer at a given pressure differential. The size of thepassageways directly affects the impedance of air that may flow acrossthe passive radiator 232. As the passageways decrease in size, the airflow for a given pressure differential, and therefore, the heat transferefficiency of the cooling assembly 230, will also decrease. Thus, onedesign goal is to balance the surface area of the passive radiator 232against the size of the passageways and resultant air flow and heattransfer efficiency. In this way, different operating conditions may bemet by adjusting the proportions of the passive radiator 232 to therequirements of the specific application and environment.

As noted above with respect to FIG. 3, the passive radiator 232 alsoprovides shock and vibration protection. These shock and vibrationaspects of the passive radiator 232 are also dependent on theproportions of each “segment”. It may be necessary to balance theapplication's need for shock and vibration protection against theoperating temperature requirements. Typically, it is required thatsystems operate at ambient temperature extremes above 50 degreesCelsius. Maximum chip case temperatures measured at the package arecommonly specified not to exceed 75 C. For low power devices, this iseasily achieved. For higher power devices, the thermal resistance fromthe electronics to air becomes a significant factor. In the case ofhigher power devices, a different material may be used for the passiveradiator 232 in order to improve the heat transfer to the coolingassembly 230, such as copper or carbon composite materials.

As noted above, heat spreader plate 240 is preferably formed from amaterial with a high thermal conductivity, such as aluminum.Alternatively, the heat spreader plate 240 may be formed from copper ora carbon composite in order to provide a higher thermal conductivity andimproved cooling efficiency at higher rates of airflow. Any type ofmaterial may be used for the passive radiator 232 in this alternateembodiment.

In one embodiment, heat spreader plate 240 or the passive radiator 232may be configured to conduct heat from a “hotter” exhaust side 715 ofthe air channels to a “cooler” inlet side 710, to allow the energy fluxinto the air channel to stay constant, along an axis of the heatspreader plate 240. This can be accomplished by making the heat spreaderplate relatively thicker at the inlet side 710 and thinner at theexhaust side 715. In another embodiment, a turbulence gradient may beachieved by varying the cooling assembly 230 channel capacity, or byvarying the pin density of the passive radiator 232, (if a pin-styleheat sink similar to pin-style heat sink 232 c is used,) by changing theprofile of pins, or by any other means. FIG. 7 illustrates a coolingassembly 230 with a turbulence gradient. The cooling assembly 230 has anintake 710 represented by the air-flow arrow 710 a and an exhaust 715,represented by arrow 715 a. Near the intake 710 of the cooling assembly230 the passive radiator 232 is comprised of elliptical pin fins 232 d.As air moves along the passive radiator 232 from intake 710 to exhaust715, along a direction indicated by arrow 720, the pressure drop alongthe direction 720 of airflow is increased. At the exhaust 715 end of thecooling assembly 230, the pin fins 232 e are shaped to be morecylindrical, which may be similar to the pin style heat sink 232 c.These cylindrical pin-fins 232 e induce more turbulence and thus createa higher pressure drop. The varying turbulence caused by changing thepin profile along arrow 720, tends to keep the rate of energy transferconstant, even though the temperature of the air increases from theintake 710 to the exhaust 715 of the cooling assembly 230. Thisturbulence profiling makes it easier for the heat spreader to maintainan isotherm. The thermal conductivity of the heat spreader can beincreased, usually meaning the mass can be reduced, thus allowing thestructure's resonant frequency (for flexure modes) to be increased, withno reduction in heat transfer efficiency.

The turbulence profiling described above helps maintain several chips incontact with the heat spreader plate 240 at a similar temperature. Thismay be especially helpful in the situation where high rates of airflow430 are induced by an externally generated pressure differential frominlet to exhaust. Referring back to FIG. 4, as the air flows in thedirection of arrow 430, it will be heated by passive radiator 232,thereby reducing its effectiveness in cooling the remainder of thepassive radiator 232. By designing the turbulence profile to match thechanges in airflow temperature, the temperature of the electroniccircuit 210 may be maintained. By maintaining a substantially uniformtemperature across all components in electronic circuit 210, timingvariances due to temperature variations between components may bereduced. This may be especially important if several processors areoperating in parallel.

While the above discussion focused primarily on an embodiment of theenclosure 200 which utilizes an air cooled corrugated fin passiveradiator 232 a, one skilled in the art will recognize that liquids suchas sea water or a commercial refrigerant, other gasses such as gaseousnitrogen, may be used to conduct heat away from the passive radiator232. Alternatively, there may be no liquid or gas present in the systemand thermal transfer is achieved by radiation or convection from theexternal surfaces of the enclosure. One embodiment utilizes a liquidheat exchanger, substituting fluid channels for the passive radiator232. All the mechanical benefits of the truss plate structure would beretained, and the modest increase in mass would be more than compensatedfor in heat transfer efficiency. Another embodiment puts the passiveradiator in physical contact with a cold wall in an aircraft.Additionally, heat pipes may be embedded in the heat spreader plate 240to help remove heat to an external heat exchanger. Additionally, while acorrugated fin 232 a and a triangular fin truss 232 b have proven to beadvantageous from a production and structure standpoint, one skilled inthe art will recognize that other passive radiators are alsocontemplated by this disclosure. Examples of other possible passiveradiators include punched corrugated fins, conventional fin-style heatsinks that may be coupled to the top and bottom plates 240, 234,honey-comb truss structures oriented to allow air to pass through them,or a solid metal plate with longitudinal channels or holes placedtherein.

While the invention has been particularly shown and described withreference to a preferred embodiment and various alternate embodiments,it will be understood by persons skilled in the relevant art thatvarious changes in form and details can be made therein withoutdeparting from the spirit and scope of the invention.

1. A ruggedized electronics enclosure for housing an electronic circuitcomprising: a first area configured to house the electronic circuit; acooling assembly comprising a rigid truss plate structure, rigidlycoupled to first area and thermally coupled to the electronic circuit,the first area, cooling assembly, and electronic circuit providing arigid structure that does not substantially deform in response to one ormore destructive shock events, to protect the electronic circuit againstsaid one or more destructive shock events; and wherein a first axisaligned with the length of the ruggedized enclosure is in a firstdirection.
 2. The ruggedized electronics enclosure of claim 1, whereinsaid first direction is substantially vertical.
 3. The ruggedizedelectronics enclosure of claim 1, wherein said first direction issubstantially horizontal.
 4. The ruggedized electronics enclosure ofclaim 1, wherein said first direction is substantially in any direction.5. A system for housing and protecting an electronic circuit in aruggedized electronics enclosure comprising: means for enclosing theelectronic circuit in a first area; means for externally cooling theelectronic circuit with a rigid cooling assembly; and means forrigidifying the electronics enclosure by rigidly coupling the coolingassembly comprising a rigid truss plate structure, to the first area,wherein said rigidified electronics enclosure does not substantiallydeform in response to one or more destructive shock events and a firstaxis aligned with the length of the ruggedized enclosure is in a firstdirection.
 6. The system of claim 5, wherein said first direction issubstantially vertical.
 7. The system of claim 5, wherein said firstdirection is substantially horizontal.
 8. The system of claim 5, whereinsaid first direction is substantially in any direction.